This invention relates to drum mixers and the like for aggregate and asphaltic materials such as may be used in connection with road surfacing, more particularly to such mixers for processing re-cycled road surfacing materials. This invention also relates to the method of processing of re-cycled road surfacing materials to the end of minimizing, if not eliminating, atmospheric pollution both of a particulate nature and of noxious fumes.
Drum mixers and/or dryers are well known including ones for processing or re-cycling used bituminous or asphaltic materials from existing roads and highways. U.S. Pat. No. 4,039,171, Aug. 2, 1977 and U.S. Pat. No. 4,143,972, Mar. 13, 1979 may be referred to in this connection.
Such prior drum mixers include an elongated drum that is adapted to rotate and, when in use, is mounted at a downward angle from the intake to discharge so that the processed material will move therethrough under the force of gravity. The drum is mounted on a framework supported by wheels at one end and is adapted at its other end to be attached to a tractor for towing the apparatus to the point of use.
At the intake end of the drum there is a source of high temperature gases which in the burning condition are blown into the drum and at this same end there is a conveyor arrangement for carrying the material to be heated into the drum. Essentially, at the intake end of the drum the raw materials which may be broken up pieces of old road surfacing, for example, come into contact with the flame and/or hot gases of the combustion process, are melted and carried through the drum to the discharge end for re-application to the road surface. Also at the intake area of the drum there are spirally arranged flights on the interior surface (at an angle to the drum axis) for conveying the raw material downstream. At the termination of the spiral, or angular, flights there are further mixing flights arranged parallel to the axis of the drum and cause the material the drum to tumble for mixing purposes. In this process additional asphalt, for example, may be added in order to make the desired end composition of the prescribed constituency.
In the prior devices liquid asphalt has been provided at the intake end of the drum in order to mix with the fine materials entering thereat for preventing the fines from entering into the current of heated gases and being carried through the drum to the discharge end and thus avoiding particulate contamination of the atmosphere. However, this would ordinarily cause burning of the asphalt or other similar material at the intake and thus cause noxious fumes to be emitted at the discharge end. To avoid this the prior art devices have used various schemes for reducing the temperature gradient of the hot gases and flame across the interior of the drum. These schemes have included heat dispersion screens, or grids, as in U.S. Pat. No. 4,039,171 and a completely enclosed combustion chamber having air intake holes disposed along its length and a truncated cone exit having a series of holes through which the heated gases flowed as in U.S. Pat. No. 4,143,972.
While these schemes may have solved the problems of controlling particulates and fumes into the atmosphere they, nevertheless, have the serious disadvantage of relatively low throughput or productive capacity.
Various governmental agencies, such as the Boards of Health and the Environmental Protective Agency, determine the standards of operation of drum mixers of the nature involved in this application such that the amount of the particulate matter as well as noxious fumes introduced into the atmosphere is severely limited.
The temperature gradient controlling schemes of the prior art, not only result in a relatively reduced productive capacity, but they are relatively difficult to maintain and are expensive in the first instance.
Injecting water spray into the cone of flame in a combustion chamber also is known as in U.S. Pat. No. 3,748,080, July 24, 1973. Such arrangements have been complicated and have not been suggested for drum mixers in connection with the re-cycling of bituminous road paving materials.
Accordingly, it is an object of the invention to provide an improved drum mixer of the nature discussed wherein re-cycled bituminous paving material may be processed at an increased productive capacity while at the same time minimizing the emission of particulate material and noxious fumes.
It is a further object of the invention to provide an improved drum mixer of the character described which is simple in construction, efficient in operation and simple to maintain.
Further advantages and objects of the invention will become apparent as the description proceeds.